1. Field of the Invention
Embodiments of the present invention generally relate to fiber designs for use in wavelength tunable ultra-short pulse lasers. More specifically, embodiments of the present invention relate to fiber designs for higher order mode fibers capable of soliton self frequency shifting where system utilizing such fibers has a first fiber for shifting the wavelength from a pump wavelength to a transfer wavelength and a second fiber for shifting the pulse from the transfer wavelength to an output wavelength.
2. Description of the Related Art
Soliton self frequency shifting (SSFS) in fibers with anomalous dispersion has proven to be an efficient method for wavelength shifting of ultra-short pulses (i.e., having a pulse width below about 1 ps). Anomalous dispersion is a requirement for sustaining a soliton pulse, and for wavelengths below 1300 nm, anomalous dispersion cannot be obtained in conventional single mode fibers. It is possible, however, to obtain anomalous dispersion in a wide wavelength range below 1300 nm using either index guided photonic crystal fibers, hollow core fibers, or fibers configured to propagate higher order modes (HOM).
To be suitable for some applications, e.g. for multi-photon imaging, the required pulse energy is generally in the nJ regime. Index guided photonic crystal fibers require very low pulse energies and SSFS in hollow core fibers requires very high pulse energies, rendering both unsuitable for such applications. However, HOM fibers may useful in such range of pulse energy.
The pulse energy required for a soliton pulse is given by:
  E  =                    KN        2            ⁢              DA        eff              T  where T is the soliton pulse width, N is an integer defining the soliton order, D is the dispersion coefficient of the fiber, Aeff is the effective area of the fiber and K is a constant.
HOM fibers can be designed with a specific dispersion coefficient and effective area, such that it is possible to tune the wavelength of the pulse by using SSFS. Current HOM fiber designs allowed for shifting from 1064 nm up to 1200 nm in the LP02 mode, resulting in output pulses with an energy of about 0.8 nJ using SSFS. Further shifting up to 1350 nm may be achieved using a combination of SSFS and Cerenkov generation, the latter having an output pulse energy of about 0.66 nJ. In addition, shifting from 775 nm to 850 nm using a combination of SSFS and Cerenkov generation with an output pulse energy of 0.6 nJ has been demonstrated.
FIGS. 1A and 1B provide a reference to show effective indices as well as the dispersion coefficient and effective area, respectively, for a typical prior art HOM fiber for pumping at 1060 nm which yields Cerenkov generation at 1300 nm when pumped at 1060 nm. As shown, the only crossing between the effective index of the LP02 mode and other modes happens at about 940 nm, well below operating wavelength range. Light in a HOM fiber where the light is in a mode having a mode-crossing with one or more other modes is generally not beneficial for any particular application. Such mode-crossing will likely introduce mode-coupling and distribute the power of the light between the involving modes and thereby prevent single mode operation at that particular wavelength. It is well known, the D·Aeff value for known HOM fiber designs at the pump wavelength is less than 3.0 fs.
As stated above, current HOM fiber designs have limited the pulse energy of the wavelength shifted pulse to just below 1 nJ. However, for practical applications, e.g. medical use with in-vivo measurements, pulse energies in the range 2-5 nJ are necessary. Accordingly, there is a need for an improved fiber design for wavelength tunable ultra-short pulse lasers.